This invention relates to a monochromator which employs a diffraction grating having a groove pattern formed by holography.
In a monochromator which employs a diffraction grating, various points on the inner wall of the housing of the instrument illuminated by the diffracted light become secondary sources, the light from which strikes the grating again and a part of the doubly diffracted light goes through the exit slit as stray light. To reduce such stray light it has been proposed to arrange the entrance and exit slits in the different spaces divided by a horizontal plane including the Rowland circle, which plane will be referred to as the Rowland plane.
Suppose, for example, that the entrance and exit slits are positioned above and below the Rowland plane, respectively. The diffracted light rays from the grating will hit the inner wall of the housing of the monochromator at points below the Rowland plane, so that the doubly diffracted light will strike the inner wall at points above the Rowland plane and will not come out of the exit slit positioned below the plane, with resulting decrease in the amount of stray light coming out of the exit slit.
In conventional in-plane monochromators, it has been customary to align the entrance and exit slits in the Rowland plane. If, with a conventional grating, the entrance and exit slits are mounted off the Rowland plane by the same distance on different sides of the plane, the aberrations will increase considerably. This type of mounting will be referred to as the off-plane mounting. There is an increasing demand for a grating for the off-plane mounting which has a small aberration as possible.
There are known two methods of forming diffraction gratings. In one of the two methods the grating grooves are ruled mechanically by means of the ruling engine. The other method utilizes interference of a pair of coherent light beams to form the grooves. The grating formed by the former method will be referred to as the mechanical grating while the grating formed by the latter method will be referred to as the holographic grating.
In recent years, the holographic grating has come into increasing use in grating monochromators because of the advantage that aberrations can be substantially reduced or eliminated by selecting the positions of the two recording point sources which determine the groove pattern.
If the entrance and exit slits are positioned at the same points as the two recording sources, respectively, and one of the slits is at the center of curvature of the concave grating, the light which has the same wavelength as the recording sources goes through the exit slit and the image at the exit slit has no aberration. For a constant deviation monochromator which has a grating formed by using recording sources having a wavelength within the predetermined wavelength range covered by the monochromator, the entrance and exit slits may well be adjacent to the positions of the recording sources in order to minimize aberrations within the whole wavelength range.
Therefore, it is conceived that a holographic grating for use in an off-plane monochromator is formed by using recording sources deviated from the Rowland plane toward different sides of it to reduce the aberration due to the off-plane mounting. However, it has been found out that the arrangement cannot attain the intended object.